Measuring Frequency Modulation

Measuring Frequency Modulation

Last updated: August 12, 2005

The performance test standards of frequency modulation are defined in the TIA/EIA-690 standard.

Testing 3.3.2.3 Modulation Deviation Limiting

Definition : Modulation Limiting refers to the ability of the transmitter circuits to prevent the transmitter from producing deviation in excess of rated system deviation.

The following procedure demonstrates how to measure the frequency modulation deviation limiting.

  1. Connect the mobile station's antenna output to the test set's front panel RF IN/OUT connector.
  2. Connect the mobile station's audio input to the test set's front panel AUDIO OUT and power on the test set.
  3. Press the blue SHIFT key and then the green Preset key to full reset the test set and power on the mobile station.
  4. Establish a call with the mobile station (see How Do I Set Up an AMPS Call? ).
  5. Initialize the measurement:
    • Press the Measurement selection key.
    • Turn the knob to highlight the Frequency Modulation measurement and press the knob.
  6. Press the Frequency Mod ( F1 ) key to access the Frequency Mod Setup menu.
  7. Set the measurement parameters as needed for your measurement situation by using the knob and DATA ENTRY keys.
    • Select Filter Type to be 50 Hz to 3 kHz.
    • Select Detector Type to be Peak +/- Max.

       
    NOTE
     
    For statistical measurement results, highlight the Multi-Measurement Count parameter and press the ON key or enter the number of the measurements. The number of averages is default to 10.
       

       
    NOTE
     
    If single is selected in the Trigger Arm field, press the START SINGLE key to trigger each measurement. If the Trigger Arm is set to Continuous then the measurement began executing as soon as you initialized the measurement in step 5.
       

  8. Press the Close Menu ( F6 ) key to close the Setup window.
  9. Enable audio generator in the test set.
    • Press the Instrument selection key.
    • Turn the knob to highlight the Audio Generator instrument and press the knob.
  10. Configure the audio generator output
    • Press the Frequency ( F3 ) key (on Call Control 1 of 2) to set the frequency of the generated signal to be 1 kHz
    • Press the Amplitude (F1) key (on Call Control 1 of 2) to adjust the amplitude of the generated signal until the peak FM Deviation is +/- 8 kHz.
    • A typical display is shown below
  11. Press the Amplitude ( F1 ) key to increase the audio output amplitude level by 20 dB in one step. A typical display for the measurement is shown below.
  12. Press the Frequency ( F3 ) key to vary the audio frequency between 300 Hz and 3000 Hz. Both the instantaneous peak and the steady state deviations at and after the time of increasing the signal level shall be observed. The results shall not exceed the rated system peak frequency deviation of +/-12 kHz at any audio frequency or reasonable change in input level. A typical display for the measurement is shown below.
  13. After finishing the measurements:
    • If desired, turn the instrument off by pressing the Instrument selection key, selecting Audio Generator , then Close Measurement ( F4 ) .
    • If desired, turn the measurement off by pressing the Measurement selection key, selecting Frequency Modulation, then Close Measurement ( F6 ) .

Related Topics

 

Frequency Modulation (FM) Measurement Description

Programming a Frequency Modulation (FM) Measurement

Frequency Modulation (FM) Troubleshooting